The present invention relates to the forming of a Schottky diode in silicon carbide.
1. Field of the Invention
In the field of semiconductor components, the material which is currently mainly used is silicon. To withstand very high voltages, silicon carbide is a priori preferable since silicon carbide has a breakdown voltage per unit thickness approximately 10 times greater than silicon.
However, in the present state of technologies, the processes currently used to form silicon-based components cannot be transposed to form silicon carbide (SiC)-based components. In particular, it is currently not possible in practice to perform implantations and diffusions of P-type dopants in N-type doped silicon carbide, noting that the P-type dopant currently used for silicon carbide is aluminum and that the N-type dopant is nitrogen. Indeed, an anneal for diffusion of an implantation of a P-type dopant would require temperatures on the order of 1700° C., which raises acute technological problems.
2. Discussion of the Related Art
The elementary structure of a Schottky diode is illustrated in FIG. 1. This diode is formed from a heavily-doped N-type substrate 1 on which is formed an N-type epitaxial layer 2 properly doped to have the desired Schottky threshold. On this epitaxial layer N is deposited silicon oxide 3 defining a window in which the Schottky contact is desired to be established by means of an adequate metallization 4. The rear surface of the component is coated with a metallization 5.
Such a structure has a very low breakdown voltage. Indeed, the equipotential surfaces tend to curve up to rise to the surface at the periphery of the contact area, which results, especially in the equipotential surface curving areas, in very high field values, which limit the possible reverse breakdown voltage. To avoid this disadvantage, the structure shown in FIG. 2 in which a P-type peripheral ring 6 is formed by implantation-diffusion at the periphery of the active area of the Schottky diode is conventionally used for silicon-based components. As a result, the equipotential surfaces must pass in volume under the P regions and thus have a less marked curvature. The diode breakdown voltage is considerably improved. As an example with silicon having a 1016 at./cm3 doping level, the breakdown voltage will be on the order of 10 V with no guard ring and on the order of 50 V with a guard ring.
However, as previously indicated, the forming of such a P-type guard ring is not simply implementable by implantation/diffusion in a structure formed on a silicon carbide substrate. In this case, the simple structure illustrated in FIG. 1 is not desirable either, for the same reasons as in the case of a silicon substrate.